The American GPS (Global Positioning System) and the Russian GLONASS system are both satellite-based position-fixing systems using a number of earth orbiting satellites. Although each is capable of providing a high level of position-fixing accuracy neither is capable of a grade of certainty and coverage to satisfy the needs of the most demanding of applications, such as those of the civil aviation business, in which satellite navigation receivers are used only as "secondary navigation aids". However, use of both systems together gives a much higher grade of certainty since one system can be used to check the other.
The GPS civil position-fixing service makes use of signals transmitted by a number of satellites on a common carrier frequency (LI) of 1575.42 MHz. Since each GPS satellite transmits a unique modulation code it is possible for a single receiver, tuned to the common carrier frequency, to extract data separately from the transmissions of each satellite. In contrast, the GLONASS service has satellites transmitting signals on different frequencies with a common code modulation scheme. Currently, the L1 group of channels comprises 24 channels in the range 1602.5625 MHz to 1615.5000 MHz.
Naturally, it is possible to equip users each with one GPS receiver and one GLONASS receiver, or to build a receiver with largely separate frequency synthesizer and intermediate frequency stages for GPS and GLONASS, but both approaches are costly in comparison with the provision of a single GPS receiver, for example.
It is an object of the present invention to produce a receiver which is capable of receiving signals from different transmission systems having substantially unrelated frequency characteristics and which uses common components for receiving the signals from the different systems to reduce cost.